The present invention is directed to an improved drive coupling. More particularly, the present invention is directed to a frictional drive coupling with improved isolation characteristics resulting from radial softness.
A large number of drive couplings are on the market today. The particular application envisioned herein requires a radially soft coupling to avoid transmission of a large amount of vibration across the coupling. Specifically, in marine drive systems in which the final drive unit is attached to the hull in a rigid, non-steering manner, a radially soft coupling of the type described herein would be required.
Certain of the previously disclosed couplings depict certain of the features contemplated in the present invention but none of the devices of which Applicant is aware embody all of the relevant features. For example, an LCD coupling manufactured by the assignee of the present invention has the capability to provide overload protection by means of a compression slip fit between the elastomer and the housing. Yet, an LCD coupling would be unduly stiff in the radial direction to provide the desired isolation. A conventional soft shear coupling might provide adequate isolation but would lack the overload protection and, because such a coupling lacks significant precompression, the durability of a comparably sized coupling is wanting. Finally, a fully bonded coupling of which Applicant is aware has a number of structural similarities to the coupling of the present invention but lacks many of the performance characteristics thereof.
The present coupling places the elastomer under a significant precompression (on the order of at least 15-20% of its radial dimension). This precompression provides an axial frictional engagement between the elastomer and one of the housing or hub while substantially reducing lateral rigidity. This reduced lateral rigidity provides the desired radial softening and, additionally, provides torsional softness.
An example of a drive system in which the coupling of the present invention could be well utilized is a right-angle marine waterjet (or conventional propeller) drive having a vertical crankshaft. The vertical crankshaft drives the horizontally positioned shaft of the waterjet or propeller through a bevel gear set. The coupling of the present invention will normally be inserted in the crankshaft on a level which is approximately that of the engine mounts. The mounts will normally allow a certain side sway of the engine and the radial stiffness of the present coupling is designed to accommodate that lateral motion.
The fact that the engine's center of gravity is well above the plane of the engine mounts (and coupling), will produce a system which is modally coupled. There will be at least two, and possibly five, modes of vibration directed radially across the coupling. Engine mounting systems utilizing radially stiff couplings will typically have vibrational resonances near the normal cruise speed range resulting in ride discomfort and high equipment wear/short equipment life. The radial softness of the coupling of the present invention will lower the resonances to a frequency which is below the engine's idle speed.
A conventional coupling would have an elastomer with a radial spring rate expressed in lb/in, that is ten to twenty times the design torque of the coupling expressed in lb-in. The operational radial spring rate of the coupling of the present invention expressed in lb/in, is in the range of between one half and three times the magnitude of said design torque level expressed in lb-in., and, more preferably, having a magnitude that is approximately equal to that of the design torque.
Various other features, advantages and characteristics of the present invention will become apparent after a reading of the following specification.